Wind energy has been used for centuries for a variety of useful purposes including grinding grain and pumping water. Recently, there has been extensive research and development worldwide in technology to use wind to generate electricity. Generating electricity from wind power does not result in the emission of carbon dioxide, hydrocarbons, carbon monoxide, particulates or other harmful compounds. Wind energy is, therefore, an attractive alternative to at least a portion of the power generated by burning fossil fuels in conventional power plants. The use of wind energy also reduces the need for coal mining which can be hazardous to miners and harmful to the environment.
There has been a continuing need and desire for improvements in wind driven power generators, including the desire to overcome the shortcomings of conventional power generators while also providing a generator which is efficient and physically compact. This increasingly competitive source of energy is steadily providing a growing share of worldwide electricity. Significant numbers of these wind turbines have been located in particular areas with high average wind speeds to form wind farms with considerable generating capability. Wind turbines have also been used to generate electricity in off-grid applications such as remote sites.
Traditional wind turbines are typically mounted on tall towers. The towers are often placed in open fields or along a ridgeline. In addition to accessing higher wind speeds, the height of traditional wind turbines reduces or avoids risk to people, livestock, and wildlife that may be on or near the ground. But towers are expensive to build and, at least in some cases, their height may be objectionable, for example, for obstructing a view. Property owners in the vicinity of these wind turbines also have been known to object to the noise caused by the large rotating blades. Many of these traditional wind turbines have blades over 40 meters long, meaning the diameter of the rotor is over 80 meters, mounted on towers 80 meters tall. Land for the wind farm has to be purchased or leased, and transmission line easements have to be obtained from the wind farm to the existing transmission power grid. As a result, the development time is long and costs are very high. Because of these restrictions, many new wind farms cannot be built for several years.
Thus, because of the problems associated with such traditional wind farms, much current research has been devoted to smaller wind turbines. While it is possible to create turbines with a wide range of blade lengths, much recent development has been devoted to turbines with smaller blade lengths than those found in traditional wind turbines. These smaller turbines can be mounted on the roofs of buildings or on poles, which are only a fraction of the height of traditional wind turbine towers. However, typical small wind driven turbines are relatively inefficient, often only converting a small fraction of the wind's kinetic energy into usable electrical power. When these smaller wind turbines have the blades mounted within a housing, or shroud, the designs allow for greater power extraction out of the wind, compared to prior art open designs. Examples of such wind turbines are found in U.S. Pat. Nos. 7,218,011, 4,204,799, 4,075,500, 6,655,907 and 6,887,031, the disclosures of which are hereby incorporated by reference herein. These smaller scale wind turbines may be mounted on lower poles, such as at a height of 10 meters, or may be mounted on the top of buildings. Thus, the smaller turbines are less expensive to build, and create less of an impact on the environment compared to the traditional larger turbines. A small scale wind turbine is needed which is highly efficient and which retains the other advantages of wind power generation.